In industry the use of metal products manufactured by compacting and sintering iron-based powder compositions is becoming increasingly widespread. The quality requirements of these metal products are continuously raised, and as a consequence new powder compositions having improved properties are developed. One of the most important properties of the final, sintered products is the density and dimensional tolerances, which have to be consistent. Problems with size variations in the final product often originates from inhomogenities in the powder mixture to be compacted. These problems are especially pronounced with powder mixtures including pulverulent components, which differ in size, density and shape, a reason why segregation occurs during the transport, storage and handling of the powder composition. This segregation implies that the composition is non-uniformly composed, which in turn means that parts made of the powder composition are differently composed and consequently have different properties. A further problem is that fine particles, particularly those of lower density such as graphite, cause dusting during the handling of the powder mixture.
The small particle size of additives also create problems with the flow properties of the powder, i.e. the capacity of the powder to behave as a free-flowing powder. An impaired flow manifests itself in increased time for filling a die cavity with powder, which means lower productivity and an increased risk of variations in density in the compacted component, which may lead to unacceptable deformations after sintering. Further, in order to eject the compacted component from the die, minimize the wear of the die surface and to obtain parts having good surface finish without scratches it is essential that the force required to eject the component from the die is low.
Attempts have been made at solving the problems described above by adding different binding agents and lubricants to the powder composition. The purpose of the binder is to bind firmly and effectively the small size particles of additives, such as alloying components, to the surface of the base metal particles and, consequently, reduce the problems of segregation and dusting. The purpose of the lubricant is to reduce the internal and external friction during compaction of the powder composition and above all to reduce the force required to eject the finally compacted product from the die.
Various organic binding agents have been developed see e.g. U.S. Pat. Nos. 4,483,905 (Engstrom), 4,676,831 (Engstrom) 4,834,800 (Semel), 5,298,055 (Semel), 5,290,336 (Luk), 5,368,630 (Luk). The U.S. Pat. No. 5,480,469 (Storstrom) provides a brief review of the use of binding agents in the powder metallurgy industry.
In the recently published patent publication WO 2005/061157 a binding/lubricating combination of polyethylene wax and ethylene bisstearamide is disclosed. In the powder composition used for compaction, the polyethylene wax is present as a layer or coating on the iron or iron-based particles and binds the alloying element particles and the ethylene bisstearamide particles to the iron or iron-based particles. It is preferred that the composition also includes a fatty acid and a flow agent. A good combination of AD, flow, bonding and lubrication properties for the powder metallurgical composition, containing a binding/lubricating combination including the polyethylene wax and ethylene bisstearamide is achieved when the mean molecular weight of the polyethylene wax is between 500 and 750.
It has now been found that iron-based compositions having remarkably improved apparent density and also improved flow, can be obtained if fatty alcohols are used instead of polyethylene wax. All in all it has been found that fatty alcohols in combination with flow agents give interesting results as regards apparent density and flow. The apparent density is essential for the tool design. A powder with low apparent density needs higher filling height which results in unnecessarily high pressing tools, and this in turn will result in longer compaction and ejection strokes. As previously mentioned the flow is important for the productivity. It has also unexpectedly been found that when the new powder metal compositions, which include fatty alcohols as a binder and a flow agent, are compacted, the obtained green compacts have excellent weight stability, i.e. low weight scatter within a set of green compacts. This property is naturally of outmost importance for the production of high performance product.
Fatty alcohols have been mentioned in the patent literature in connection with lubrication in the U.S. Pat. No. 3,539,472. Specifically this patent teaches that small amounts of fatty alcohols can be included in lubricants mainly consisting of amides or diamides. The patent does not concern bonded mixtures.
Also the Japanese patent application 04-294 782, publication number 06-145701 mentions that fatty alcohols can be used as lubricants. Specifically mentioned are C30 alcohols, C50 alcohols and C60 alcohols. The application text also mentions higher fatty alcohols as binders.